Method for manufacturing thermal interface sheet

ABSTRACT

A thermal interface sheet includes a peripheral portion, in a surface direction, configured to have a melting point higher than the melting point of a central portion in the surface direction.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of application Ser. No. 14/524,047,filed on Oct. 27, 2014, which claims the benefit of priority of theprior Japanese Patent Application No. 2013-227997, filed on Nov. 1,2013, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a thermal interfacesheet configured to dissipate heat from a semiconductor elementeffectively and a processor including a thermal interface layer formedfrom the above-described thermal interface sheet.

BACKGROUND

In recent years, in order to cope with an increase in an amount of heatgeneration along with enhancement in performance of a semiconductordevice, high thermal conduction of a central processing unit (CPU)package structure is realized by using solder serving as a thermalinterface material (TIM) between a large scale integration (LSI) and aheat spreader.

For example, a composite material including a thermally conductive metaland silicone particles in the above-described thermally conductive metal(for example, refer to Japanese National Publication of InternationalPatent Application No. 2010-539683), a complex including a homogeneouslydispersed material of an indium metal and at least one type of ceramicmaterial and having a thermal conductivity of at least 80 W/mK (forexample, refer to Japanese Laid-open Patent Publication No.2009-161850), and a TIM serving as a phase change portion composed of atleast one type of low-melting point metal selected from the groupconsisting of Ga, In, and Sn or an alloy containing the above-describedat least one type of low-melting point metal (for example, refer toJapanese Laid-open Patent Publication No. 2007-335742) have beenproposed as the above-described TIM.

In general, it is known that voids are generated at a junction interfaceof soldering. In the case where the TIM is used, voids lead to reductionin the cooling efficiency and high-temperature irregularity of a deviceand, therefore, a junction with reduced voids is desired.

However, with respect to the thermal interface materials (TIMs) in therelated art including the above-described technologies which have beenproposed already, a junction with reduced voids has not been realizedbecause removal of generated voids from the junction surface isdifficult.

SUMMARY

According to an aspect of the invention, a thermal interface sheetincludes a peripheral portion, in a surface direction, configured tohave a melting point higher than the melting point of a central portionin the surface direction.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic top view illustrating an example of a thermalinterface sheet;

FIG. 1B is a sectional view of the section taken along a line IB-IBillustrated in FIG. 1A;

FIG. 2A is a schematic top view illustrating another example of thethermal interface sheet;

FIG. 2B is a sectional view of the section taken along a line IIB-IIBillustrated in FIG. 2A;

FIG. 3A is a schematic top view illustrating another example of thethermal interface sheet;

FIG. 3B is a sectional view of the section taken along a line IIIB-IIIBillustrated in FIG. 3A;

FIG. 4A is a schematic top view illustrating another example of thethermal interface sheet;

FIG. 4B is a sectional view of the section taken along a line IVB-IVBillustrated in FIG. 4A;

FIG. 5 is a schematic perspective view of an example of a thermalinterface block to produce a thermal interface sheet; and

FIG. 6 is a schematic sectional view of an example of a disclosedprocessor.

DESCRIPTION OF EMBODIMENTS

Thermal Interface Sheet

A disclosed thermal interface sheet includes a peripheral portion, in asurface direction of the above-described thermal interface sheet,configured to have a melting point higher than the melting point of acentral portion in the above-described surface direction.

The above-described thermal interface sheet is favorably used forjoining a semiconductor element and a heat spreader.

The above-described thermal interface sheet is a sheet having excellentthermal conductivity and may be referred to as a thermally conductivesheet.

Preferably, the melting point of the above-described thermal interfacesheet may increase stepwise from the central portion in the surfacedirection of the above-described thermal interface sheet toward theperipheral portion in the surface direction of the above-describedthermal interface sheet.

Preferably, the melting point of the above-described thermal interfacesheet may increase little by little from the central portion in thesurface direction of the above-described thermal interface sheet towardthe peripheral portion in the surface direction of the thermal interfacesheet.

The material for the above-described thermal interface sheet is notspecifically limited and may be selected appropriately in accordancewith the purpose, although it is preferable that solder be contained.

The above-described solder is not specifically limited and may beselected appropriately in accordance with the purpose. For example,In—Ag solder, Sn—Cu solder, Sn—Ag—Cu solder, Sn—Ag—Cu—Bi solder, and thelike are mentioned.

The above-described In—Ag solder may have various melting pointsdepending on, for example, the composition ratio of In to Ag, asdisclosed in Table 1.

TABLE 1 Melting point In-5Ag 280° C. In-10Ag 231° C. In-7Ag 200° C.In-5Ag 160° C. In-3Ag 141° C.

As for the above-described solder, it is preferable that the solder beconfigured to contain In because In is mild and has excellent thermalconductivity. Usually, in many cases, the semiconductor element containsSi as a primary constituent component and the heat spreader contains Cuas a primary constituent component. Here, the thermal expansioncoefficient of Si is 3 ppm/° C. and the thermal expansion coefficient ofCu is 17 ppm/° C. There is a difference in thermal expansion coefficientand, thereby, the thermal interface sheet may undergo thermal fatiguebecause of repetition of switching. In order to reduce theabove-described thermal fatigue, it is preferable that In which is mildand capable of absorbing a thermal stress be used as the constituentcomponent of the thermal interface sheet.

The above-described thermal interface sheet may be formed by usingsolder alloys configured to have different melting points depending onthe composition ratio.

The average thickness of the above-described thermal interface sheet isnot specifically limited and may be selected appropriately in accordancewith the purpose. For example, 100 μm to 1 mm is mentioned.

The size and the shape of the above-described thermal interface sheetmay be selected appropriately in accordance with sizes, shapes, and thelike of a semiconductor element and the like to be joined.

An example of the above-described thermal interface sheet will bedescribed with reference to drawings.

An example of the above-described thermal interface sheet is illustratedas FIG. 1A and FIG. 1B. FIG. 1A is a schematic top view of a thermalinterface sheet 10. FIG. 1B is a sectional view of the section takenalong a line IB-IB illustrated in FIG. 1A.

The thermal interface sheet 10 illustrated in FIG. 1A and FIG. 1B iscomposed of four types of solder configured to have different meltingpoints and includes a first solder region 11, a second solder region 12,a third solder region 13, and a fourth solder region 14. The meltingpoints of the individual solder regions satisfy the relationshiprepresented by the first solder region 11<the second solder region12<the third solder region 13<the fourth solder region 14. That is, themelting point of the thermal interface sheet 10 illustrated in FIG. 1Aand FIG. 1B increases stepwise from the central portion in the surfacedirection of the thermal interface sheet 10 toward the peripheralportion in the surface direction of the thermal interface sheet 10.

The first solder region 11, the second solder region 12, and the thirdsolder region 13 are concentric circles having different sizes in thetop view.

Another example of the above-described thermal interface sheet isillustrated as FIG. 2A and FIG. 2B. FIG. 2A is a schematic top view of athermal interface sheet 20. FIG. 2B is a sectional view of the sectiontaken along a line IIB-IIB illustrated in FIG. 2A.

The thermal interface sheet 20 illustrated in FIG. 2A and FIG. 2B iscomposed of four types of solder configured to have different meltingpoints and includes a first solder region 21, a second solder region 22,a third solder region 23, and a fourth solder region 24. The meltingpoints of the individual solder regions satisfy the relationshiprepresented by the first solder region 21<the second solder region22<the third solder region 23<the fourth solder region 24. That is, themelting point of the thermal interface sheet 20 illustrated in FIG. 2Aand FIG. 2B increases stepwise from the central portion in the surfacedirection of the thermal interface sheet 20 toward the peripheralportion in the surface direction of the thermal interface sheet 20.

The first solder region 21, the second solder region 22, the thirdsolder region 23, and the fourth solder region 24 are squares havingdifferent sizes in the top view.

Another example of the above-described thermal interface sheet isillustrated as FIG. 3A and FIG. 3B. FIG. 3A is a schematic top view of athermal interface sheet 30. FIG. 3B is a sectional view of the sectiontaken along a line IIIB-IIIB illustrated in FIG. 3A.

The thermal interface sheet 30 illustrated in FIG. 3A and FIG. 3B iscomposed of four types of solder configured to have different meltingpoints and includes a first solder region 31, a second solder region 32,a third solder region 33, and a fourth solder region 34. The meltingpoints of the individual solder regions satisfy the relationshiprepresented by the first solder region 31<the second solder region32<the third solder region 33<the fourth solder region 34. That is, themelting point of the thermal interface sheet 30 illustrated in FIG. 3Aand FIG. 3B increases stepwise from the central portion in the surfacedirection of the thermal interface sheet 30 toward the peripheralportion in the surface direction of the thermal interface sheet 30.

The first solder region 31, the second solder region 32, the thirdsolder region 33, and the fourth solder region 34 are regular hexagonshaving different sizes in the top view.

Another example of the above-described thermal interface sheet isillustrated as FIG. 4A and FIG. 4B. FIG. 4A is a schematic top view of athermal interface sheet 40. FIG. 4B is a sectional view of the sectiontaken along a line IVB-IVB illustrated in FIG. 4A.

In the thermal interface sheet 40 illustrated in FIG. 4A and FIG. 4B,solder is present, where the melting point varies without exhibiting aclear boundary. That is, the melting point of the thermal interfacesheet 40 illustrated in FIG. 4A and FIG. 4B increases little by littlefrom the central portion in the surface direction of the thermalinterface sheet 40 toward the peripheral portion in the surfacedirection of the thermal interface sheet 40.

In FIG. 4A and FIG. 4B, the gradual change in the melting point of thethermal interface sheet 40 is expressed by stippling, where as thedensity of dots becomes low, the melting point becomes relatively low,and as the density of dots becomes high, the melting point becomesrelatively high.

A method for manufacturing the above-described thermal interface sheetis not specifically limited and may be selected appropriately inaccordance with the purpose. Examples include a method in which athermal interface block 50 illustrated as FIG. 5 is produced, and thethermal interface block 50 is cut along the alternate long and shortlines illustrated in FIG. 5. A method for cutting is not specificallylimited and may be selected appropriately in accordance with thepurpose. For example, a method in which cutting is performed with anultrasonic cutter is mentioned.

The thermal interface sheet block 50 illustrated as FIG. 5 includes afirst solder region 51 in a central region and includes a second solderregion 52 and a third solder region 53 in that order in the peripherythereof in a y-z plane. The melting points of the individual solderregions satisfy the relationship represented by the first solder region51<the second solder region 52<the third solder region 53.

A method for manufacturing the thermal interface sheet 50 illustrated asFIG. 5 is not specifically limited and may be selected appropriately inaccordance with the purpose. Examples include the following methods (1)to (4).

Method (1)

A method in which sheet-shaped second solder (solder to constitute thesecond solder region 52) is wound around quadratic prism-shaped solderformed from first solder to constitute the first solder region 51, andsheet-shaped third solder (solder to constitute the third solder region53) is wound around them. In this method, when each sheet is wound,preferably, hot press is performed in such a way that each solder regionis melted and is joined.

Method (2)

Quadratic prism-shaped solder formed from first solder to constitute thefirst solder region 51 is immersed into molten second solder (solder toconstitute the second solder region 52), so that the second solderregion 52 is formed around the first solder region 51. Subsequently, thequadratic prism-shaped solder provided with the second solder region 52is immersed into molten third solder (solder to constitute the thirdsolder region 53), so that the third solder region 53 is formed.

In immersion into the molten solder, it is preferable that the quadraticprism-shaped solder to be immersed is cooled sufficiently. Consequently,immersion may be performed while melting of the quadratic prism-shapedsolder to be immersed is reduced.

According to this method, mainly, a thermal interface block may beformed, where the melting point increases stepwise from the centralportion toward the peripheral portion in the y-z plane.

On the other hand, it is also possible to make the interfaces betweenthe individual solder regions unclear by adjusting the temperature ofthe molten solder and the degree of cooling of the quadraticprism-shaped solder appropriately and, thereby, form a thermal interfaceblock, where the melting point increases little by little from thecentral portion toward the peripheral portion in the y-z plane.

Method (3)

A quadratic prism-shaped core material having a sufficiently highmelting point is immersed into molten second solder (solder toconstitute the second solder region 52), so that the second solderregion 52 is formed around the above-described quadratic prism-shapedcore material. Subsequently, the above-described quadratic prism-shapedcore material provided with the second solder region 52 is immersed intomolten third solder (solder to constitute the third solder region 53),so that the third solder region 53 is formed. Thereafter, the corematerial is removed from the thermal interface block provided with thethird solder region 53, first solder (solder to constitute the firstsolder region 51) is poured into the resulting central portion, andcooling is performed.

Method (4)

The inside surface of hollow quadratic prism-shaped solder made fromthird solder (solder to constitute the third solder region 53) is coatedwith molten second solder (solder to constitute the second solder region52) and cooling is performed, so that the second solder region 52 isformed on the inside surface of the third solder region 53.Subsequently, molten first solder (solder to constitute the first solderregion 51) is poured on the inside of the second solder region 52, andcooling is performed.

In the above-described method, the quadratic prism-shaped solder isused. However, the shape is not specifically limited and may be selectedappropriately in accordance with the purpose. The shape of a circularcolumn may be employed.

Processor

A disclosed processor includes at least a semiconductor element, athermal interface layer, and a heat spreader and further includes othermembers, as occasion calls.

Semiconductor Element

The above-described semiconductor element is not specifically limitedinsofar as a circuit surface is included and may be selectedappropriately in accordance with the purpose. Examples includeintegrated circuits and large scale integrated circuits.

Thermal Interface Layer

The above-described thermal interface layer is formed from the disclosedthermal interface sheet.

The above-described thermal interface layer is disposed on the surfaceopposite to the above-described circuit surface of the above-describedsemiconductor element.

Heat Spreader

The above-described heat spreader is not specifically limited and may beselected appropriately in accordance with the purpose. The heat spreaderis formed from, for example, a material having good thermal conductionperformance. The above-described heat spreader may be formed from Cu,Al, or a composite material based thereon, although oxygen-free-copperis preferable. The above-described heat spreader is disposed on theabove-described thermal interface layer.

The size and the shape of the above-described heat spreader may beselected appropriately in accordance with the size, the shape, and thelike of the above-described processor.

Examples of the above-described processor include a central processingunit (CPU), a digital signal processor (DSP), and a graphics processingunit (GPU).

An example of the above-described processor will be described withreference to FIG. 6.

FIG. 6 is a schematic sectional view of an example of the processor.

The processor illustrated as FIG. 6 includes a package substrate 1, asemiconductor chip 2 serving as the above-described semiconductorelement, a thermal interface layer 3, a heat spreader 4, an underfillresin 5, and a stiffener 6.

The semiconductor chip 2 is mounted on the central portion of thepackage substrate 1. Connection bumps 1 a are disposed on the back ofthe package substrate 1.

The stiffener 6 in the shape of a frame is fixed on the chip-mountingsurface of the package substrate 1 in such a way as to surround themounting area of the semiconductor chip 2. The stiffener 6 is used as areinforcement to reduce harmful deformations, e.g., warp, of the packagesubstrate 1 or breakage when an external force is applied to thepackage, and Cu or a stainless steel having a thermal expansioncoefficient close to the thermal expansion coefficient of the packagesubstrate 1 is used.

The semiconductor chip 2 is connected to connection lands on the packagesubstrate 1 through the use of electrodes 2 a disposed on the circuitsurface. As for the electrode material, Sn—Ag solder, Sn—Pb solder, orthe like is used.

In order to reduce break due to thermal fatigue of the junction portionwith the package substrate 1, the underfill resin 5 having an insulatingproperty is filled in the junction portion of the electrodes 2 a withthe package substrate 1. As for the underfill resin 5, a synthetic resincontaining an epoxy resin as a primary component and having a thermalexpansion coefficient of about 20 ppm/° C. to 80 ppm/° C. is used.

The thermal interface layer 3 formed from the thermal interface sheet isdisposed between the surface opposite to the circuit surface of thesemiconductor chip 2 and the heat spreader 4. The thermal interfacelayer 3 is formed by heating and melting the thermal interface sheet andis configured to solder-joining the semiconductor chip 2 and the heatspreader 4 and transfer the heat of the semiconductor chip 2 from thesemiconductor chip 2 to the heat spreader 4.

In the case where the thermal interface sheet and the heat spreader 4are stacked on the semiconductor chip 2 in that order, slight gaps(voids) are generated between the semiconductor chip 2 and the thermalinterface sheet and between the thermal interface sheet and the heatspreader 4, and air (bubble (void)) is present there.

As for the thermal interface sheet in the related art, if heating isperformed in that state, the thermal interface sheet is melted all atonce. Therefore, the air present between the semiconductor chip 2 andthe thermal interface sheet and between the thermal interface sheet andthe heat spreader 4 is not transferred easily and remains as bubbles atthe interface between the semiconductor chip 2 and the thermal interfacesheet and at the interface between the thermal interface sheet and theheat spreader 4. The thermal conductivity of the air is low, so that ifthe air remains at the interface between the semiconductor chip 2 andthe thermal interface sheet and at the interface between the thermalinterface sheet and the heat spreader 4, transfer of the heat from thesemiconductor chip 2 to the heat spreader 4 is reduced.

On the other hand, in the case where the above-described disclosedthermal interface sheet is used, when heating is performed, the thermalinterface sheet begins to melt from the central portion in the surfacedirection. Consequently, when the molten central portion of the thermalinterface sheet wets and spreads to the semiconductor chip 2 and theheat spreader 4, bubbles present at the interface between thesemiconductor chip 2 and the thermal interface sheet and at theinterface between the thermal interface sheet and the heat spreader 4are pushed to the outside in the surface direction. As the thermalinterface sheet is melted from the central portion toward the peripheralportion in the surface direction of the thermal interface sheet, bubblesare pushed to the outside in the surface direction. As a result, the airmay be removed from the interface between the semiconductor chip 2 andthe thermal interface sheet and the interface between the thermalinterface sheet and the heat spreader 4.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A method for manufacturing a thermal interfacesheet, comprising: preparing a first solder region having a quadraticprism shape and being formed from a first solder; immersing the firstsolder region into a second solder which is melted so as to form asecond solder region having a quadratic prism shape around the firstsolder region; immersing the first solder region and the second solderregion into a third solder which is melted so as to form a third solderregion having a quadratic prism shape around the second solder region;and cutting the first solder region, the second solder region and thethird solder region, wherein a third melting point of the third solderis larger than a second melting point of the second solder and thesecond melting point is larger than a first melting point of the firstsolder.
 2. The method according to claim 1, wherein the first solderregion, the second solder region and the third region are providedconcentrically.
 3. The method according to claim 1, wherein the firstsolder region is cooled before being immersed into the second solder,and the first solder region and the second solder region are cooledbefore being immersed into the third solder.
 4. The method according toclaim 1, wherein the first solder region, the second solder region andthe third solder region contain one of an In—Ag solder, a Sn—Cu solder,a Sn—Ag—Cu solder, and a Sn—Ag—Cu—Bi solder.